1. Field of the Invention
The present invention relates to a scanning optical device suitably used in an image recording apparatus and an image reading apparatus, and, more particularly, to a scanning optical device exhibiting high resolution with a reduced cost.
2. Description of the Prior Art
Hitherto, an apparatus for compensating for the imaging position error of a scanning optical device has been exemplified by devices, such as that disclosed in Japanese Patent Laid-Open No. 59-16603 which comprises a light source moving actuator which moves a piezo electric device to which a light source or a lens is fastened. Another apparatus has been disclosed in Japanese Patent Laid-Open No. 61-25367 in which the quantity of the deviation between a main scanning line and a spot mark is previously stored so that the intensity of a laser which serves as a light source is controlled in accordance with the stored quantity. A still further apparatus has been disclosed in Japanese Patent Laid-Open No. 61-185716 in which an adjustable focusing mechanism operates in such a manner that the laser beam is always linearly imaged on a polygonal mirror surface.
However, the following problems are present in the above-described conventional apparatuses:
According to the first conventional apparatus, the position of the optical element can be controlled by controlling the voltage applied to the piezo electric device so that a good spot imaged state can be realized over the scanning lines on the surface to be scanned.
However, since the voltage level to be applied to the piezo electric device periodically changes, it is very difficult to satisfactorily compensate the scanning lines.
Furthermore, it is difficult to use the first conventional apparatus with the pre-objective scanning optical system which employs an f..theta. lens since a description is provided for only the post-objective scanning optical system which does not comprise the f..theta. lens.
According to the second conventional apparatus, a compensation method in which the quantity of the deviation between the main scanning line and the spot mark is stored is employed. However, the spot mark is not compensated since the apparatus is arranged in such a manner that the intensity of the laser beam is controlled in accordance with the stored quantity of deviation and the diameter of the recording spot diameter on the surface to be scanned is compensated. Therefore, a high resolution device cannot be easily realized. Furthermore, according to the above-described compensation method, the diameter of the recording spot in the portion in which the quantity of the deviation is large can be made the same as that in the portion in which the quantity of the deviation is small. However, since the exposing energy density is different between the above-described two portions, the recording density becomes nonuniform. As a result, it is very difficult to provide acceptable halftone images.
According to the third conventional apparatus, the curvature of field in the sub-scanning direction (in the sagittal direction) due to the rotation of the polygon mirror can be perfectly compensated. However, the third conventional apparatus cannot compensate the curvature of field in the main scanning direction (in the meridional direction, that is, in a direction perpendicular to the sub-scanning direction perpendicular to the scanning surface which is formed by the scanning beam with time which has been deflected by the rotation of the polygon mirror) and that in the sub-scanning direction (sagittal direction) of the optical system. Therefore, the third conventional apparatus must employ, as the f..theta. lens thereof, an optical system the curvature of field of which has been previously and satisfactorily compensated in both the meridional and the sagittal directions. Therefore, the overall cost is raised and the design of the f..theta. lens becomes very difficult when high resolution is desired to be achieved.
Therefore, an object of the present invention is to provide a scanning optical device capable of overcoming the above-described problems experienced with the conventional structures and satisfactorily for the quantity of the deviation of the scanning optical system.